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1987 - 25hp occasional steam issue - suggestions?

S

sadbassa

New member
Hi,

I have a 1987, 25hp, two stroke Mariner outboard attached to the back of a 22ft inland cruiser.

I recently took the boat out and was running the outboard at about 3/4 revs and after about 10 minutes the engine began to pump out quite a lot of steam. I cut the engine and used my back up to get the boat back to it's mooring. I then checked and replaced the impeller (which was in good condition) and change the thermostat which was obviously old and corroded.

When complete I took the boat out and it seemed ok, I was on the boat again this weekend and took her out and I ran the engine at half power for about an hour and a half with no problem running at about 5 knots, there was a very occasional puff of steam, but nothing more than you would expect from splashes on hot parts.

On the way back to the mooring, I decided to test the engine again at higher revs feeling confident the repair had worked and with a couple of minutes the steam started coming from the engine again, not as bad as previously but it was still noticeable. I cut the engine power right back for the short distance back and the engine ran fine, didn't appear to be overheating but still pumped out a bit of steam.

Does anybody have any ideas or any suggestions as to what the problem may be? Or is an amount of steam normal in these engines? I have checked the water intake and it is clear and free from blockages.

The engine starts on the key, runs smoothly and is powerful but I am concerned it is going to seize up if there is a underlying problem with it.

Thanks

Steve
 
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Hard to say if it's overheating by steam alone. I suggest pulling the cowling and running it with it off, then checking the left side exhaust plate with a temp gun (or your hand--you should be able to keep your hand on the plate).

Jeff

PS: That's a lot of boat for a 25 hp motor to push. It might simply be way over-propped for the service.
 
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Cracked head. Had this same thing one time. It would go all day at half throttle or less with no problem, open it up and it would steam. Put a new head on it, just like new still, that was 15 years ago. Run mostly in salt water. Believe that sand stuck in the water passages trapped some water and it froze to crack the head.
 
I had a similar problem that had kind of an odd-ball cause. Had the motor off the boat at a mechanic's shop for some work. He changed the tilt setting (probably to run it in a barrel?) and I didn't notice. Put the motor on and went for a test spin. Serious overheating at high speed. Limped home, checked everything - nothing noticeably wrong - except I noticed the tilt angle looked odd. Took it out again, same load, same trim. Wound it up and, sure enough, the tilt angle was forcing the bow down and stern up to the point where the water intake was coming right out of the water. Changed the tilt angle and no more problem. On the cylinder head, I believe we have the same motor (mine's a '82 Japanese made 30 HP) - there is definitely a cylinder head.
 
When you changed the impeller did you take a really good look at the waterpump cover?

The seals are often overlooked during an impeller change, particularly the "oil seal" around the driveshaft and the seal where the water tube enters the cover. You may be losing pressure at the pump itself (blowing water simply into the leg) such that at higher rpms you are simply not getting enough water up the tube to properly "feed" the powerhead - resulting in steam.

There are a few potential issues further (up stream) but that is where I would start my troubleshooting.
 
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